A super-high pressure discharge lamp in which at least 0.2 mg/mm3 mercury is enclosed in an interior of a discharge container and configured to increase in pressure to 200 atmospheric pressure or higher at the time of lighting is used for a lamp for a light source for projectors or the like. The high pressure discharge lamp as described above is known to maintain a stable lighting state by forming a projection on a distal end surface of an electrode and retaining an arc on the projection while the lamp is lighting.
In contrast, when the high pressure discharge lamp is continuously lit for a longtime in the same state, a plurality of projections may be formed or projections and depressions may be generated on the distal end surface of the electrode. It is known that when the surface portion of the distal end of the electrode is roughened, a discharging position becomes unstable, and lowering of luminance or flickering may occur due to a displacement of the arc.
In order to solve the problem described above, Patent Literature 1 describes a system of lighting a discharge lamp including supplying a frequency selected from a range between 60 and 1000 Hz to a high pressure discharge lamp as a steady frequency, and inserting a low frequency selected in frequency from a range between 5 and 200 Hz, in number of units from 1 unit to 10 units, where one unit corresponds to the number of waves to be inserted, and in intervals of insertion from a range between 0.01 seconds and 120 seconds intermittently (cyclically) into an alternating current having the steady frequency while changing parameters of the low frequency to be selected in accordance with a change of the lighting voltage of the discharge lamp.
FIG. 11 illustrates a system of lighting a discharge lamp described in Patent Literature 1 described above.
In the same figure, the drawing (a) in the same figure illustrates a case where the lamp voltage is low, and the drawing (b) in the same figure illustrates a case where the lamp voltage is high. As illustrated in the drawing (a) in the same figure, when the lamp voltage is low, an alternating current having a frequency selected from a frequency range between 60 and 1000 Hz is supplied as an alternating current having a frequency which is basic at the time of steady lighting (basic frequency), and a frequency, which is a frequency lower than the basic frequency and selected from a range between 5 and 200 Hz is supplied at a predetermined interval. When the lamp voltage is increased, a width of the frequency lower than the basic frequency to be supplied at the predetermined interval is increased as illustrated in the drawing (b) in the same figure. Accordingly, the temperature of the projection can be maintained always at an optimal state.
In other words, according to this technology, a stable arc discharge can be formed by forming a projection at a distal end of the electrode and using the projection as an original point and, even when the lighting voltage of the lamp is changed, only the projection which is to be the original point of the arc can be generated and maintained and, simultaneously, needless projections other than the corresponding projection can be eliminated, so that elongation of the lifetime of the electrode is achieved.